All paired up with no place to go: pairing, synapsis, and DSB formation in a balancer heterozygote.

Gong WJ, McKim KS, Hawley RS - PLoS Genet. (2005)

Bottom Line:
We have utilized the LacI-GFP: lacO system to visualize the effects of FM7 on meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes.However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence X chromosomes, and this effect was greatest near a breakpoint.We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over.

Affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, United States of America.

ABSTRACTThe multiply inverted X chromosome balancer FM7 strongly suppresses, or eliminates, the occurrence of crossing over when heterozygous with a normal sequence homolog. We have utilized the LacI-GFP: lacO system to visualize the effects of FM7 on meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. Surprisingly, the analysis of meiotic pairing and synapsis for three lacO reporter couplets in FM7/X heterozygotes revealed they are paired and synapsed during zygotene/pachytene in 70%-80% of oocytes. Moreover, the regions defined by these lacO couplets undergo double-strand break formation at normal frequency. Thus, even complex aberration heterozygotes usually allow high frequencies of meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence X chromosomes, and this effect was greatest near a breakpoint. We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over.

pgen-0010067-g006: DSB Formation on the Same Stretch of SC As Paired lacO Couplets in FM7/X and X/X OocytesThe representatives from FM7/X (18A/18C) and X/X (18C/18C) oocytes are shown in this figure. Single C(3)G-positive (red) meiotic cells are shown in each row. DSBs are indicated by γ-HIS2AV staining (blue, arrows). GFP foci (green, arrows) represent the pairing FM7 and X or X and X chromosomes. Two to three optical sections of images were projected. Bars = 1 μm.

Mentions:
In this study, allelic pairs at 9B and 18C in X/X females were compared with lacO couplets at 8F/9B and 18A/18C in FM7/X females. As shown in Figure 6 and Table 3, such cases of SC stretches doubly marked with GFP foci and a γ-HIS2AV focus were observed in ~5% of the SC-positive nuclei for all four genotypes studied. Indeed, combining the data for the two nearby lacO couplets studied in FM7/X oocytes, we observed 12 cases in which the same stretch of SC was marked by both a GFP focus (or paired foci) and by a γ-HIS2AV focus out of 237 oocytes examined in FM7/X oocytes (5.1%). Similarly (again combining the data for the two allelic pairs of lacO insertions studied in X/X females), nine cases in which the same stretch of SC was marked by both a GFP focus (or paired foci) and by a γ-HIS2AV focus were observed in 190 oocytes from X/X females (4.7%). Furthermore, the average distances between a GFP focus and a γ-HIS2AV focus were not significantly different between FM7/X and X/X oocytes (p = 0.86; the average distance in X/X is 0.79 μm while the value is 0.81 μm in FM7/X). While it is not possible to compare either of these frequencies to some absolute expectation of the number of such foci per given length of SC, it is clear that there is no obvious reduction in the frequency of DSBs in balancer heterozygotes.

pgen-0010067-g006: DSB Formation on the Same Stretch of SC As Paired lacO Couplets in FM7/X and X/X OocytesThe representatives from FM7/X (18A/18C) and X/X (18C/18C) oocytes are shown in this figure. Single C(3)G-positive (red) meiotic cells are shown in each row. DSBs are indicated by γ-HIS2AV staining (blue, arrows). GFP foci (green, arrows) represent the pairing FM7 and X or X and X chromosomes. Two to three optical sections of images were projected. Bars = 1 μm.

Mentions:
In this study, allelic pairs at 9B and 18C in X/X females were compared with lacO couplets at 8F/9B and 18A/18C in FM7/X females. As shown in Figure 6 and Table 3, such cases of SC stretches doubly marked with GFP foci and a γ-HIS2AV focus were observed in ~5% of the SC-positive nuclei for all four genotypes studied. Indeed, combining the data for the two nearby lacO couplets studied in FM7/X oocytes, we observed 12 cases in which the same stretch of SC was marked by both a GFP focus (or paired foci) and by a γ-HIS2AV focus out of 237 oocytes examined in FM7/X oocytes (5.1%). Similarly (again combining the data for the two allelic pairs of lacO insertions studied in X/X females), nine cases in which the same stretch of SC was marked by both a GFP focus (or paired foci) and by a γ-HIS2AV focus were observed in 190 oocytes from X/X females (4.7%). Furthermore, the average distances between a GFP focus and a γ-HIS2AV focus were not significantly different between FM7/X and X/X oocytes (p = 0.86; the average distance in X/X is 0.79 μm while the value is 0.81 μm in FM7/X). While it is not possible to compare either of these frequencies to some absolute expectation of the number of such foci per given length of SC, it is clear that there is no obvious reduction in the frequency of DSBs in balancer heterozygotes.

Bottom Line:
We have utilized the LacI-GFP: lacO system to visualize the effects of FM7 on meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes.However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence X chromosomes, and this effect was greatest near a breakpoint.We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over.

Affiliation:
Stowers Institute for Medical Research, Kansas City, Missouri, United States of America.

ABSTRACTThe multiply inverted X chromosome balancer FM7 strongly suppresses, or eliminates, the occurrence of crossing over when heterozygous with a normal sequence homolog. We have utilized the LacI-GFP: lacO system to visualize the effects of FM7 on meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. Surprisingly, the analysis of meiotic pairing and synapsis for three lacO reporter couplets in FM7/X heterozygotes revealed they are paired and synapsed during zygotene/pachytene in 70%-80% of oocytes. Moreover, the regions defined by these lacO couplets undergo double-strand break formation at normal frequency. Thus, even complex aberration heterozygotes usually allow high frequencies of meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence X chromosomes, and this effect was greatest near a breakpoint. We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over.